Prince William prided itself as being the last Republican redoubt in Northern Virginia.

The county gained national attention for a crackdown on illegal immigrants, and its legislative delegation has some of the most strident anti-tax Republican state lawmakers. When the neighboring Loudoun County Board of Supervisors went Democratic last year, Prince William maintained its GOP dominance.

But on Nov. 4, the once reliably red county in a reliably red state went solidly for Sen. Barack Obama, signaling a political shift in Virginia politics.

Although Prince William County was crucial to Obama’s winning the state, Virginia Democrats had begun a steady march into the fast-growing outer counties before the 2008 presidential election. A shift in the county’s demographics and a crashing housing market has made the Democratic Party more attractive to Prince William residents. In making a play for the county, Obama’s campaign exploited those new political dynamics.

As a result, Prince William became a bellwether county in a presidential battleground state.

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The transformation is taking place beyond Prince William and Loudoun. Other counties on the suburban fringe are closing in, too. Even though Obama did not win Stafford, Culpeper and Fauquier counties, his vote totals were roughly 10 percentage points larger than those of the Democratic presidential candidates in 2000 and 2004.

Those counties “used to be the heart of red America; now, they are burnt orange,” state Sen. J. Chapman “Chap” Petersen (D-Fairfax) said.

Obama crossed the magical 60 percent threshold in Northern Virginia by winning among both whites and African Americans — Northern Virginia was only part of the state where Obama won white voters — as well as old and young voters, and by dominating among the region’s moderate and independent voters.

“Once you start developing an area and it continues to urbanize, the philosophy changes,” said Michael P. McDonald, a senior fellow at the Brookings Institution and an associate professor at George Mason University who studies voting patterns. “People become more receptive to government playing a role in their everyday lives.”

More importantly, they become less receptive to the idea that government should play a role in parts of their lives that they believe to be personal, and they become less receptive so simplistic appeals to fear of the other.

Republicans can still win in Prince William County, and the rest of Virginia, but they won’t do so until they realize that the voting public has changed.